Because most Android users don't know about Google Lens/their camera app and google "Barcode Scanner app" when they get their phone.

My Samsung has a built-in QR scanner, which I found out by accident.

I downloaded an app for it because it never crossed my mind it would be built into the camera app. After all I don't want to take pictures of the QR code, I want to decode it...

No idea when it was introduced. I've had an S3, S5 and now an S8 where I discovered it by accident last year. Pretty sure the S3 didn't have it.

Personally, when I installed the app, there wasn't one built in. I just still had it lying around.